The introduction of integrated circuit and semiconductor devices to motor vehicles has greatly increased the control capability available to the designer. At the same time, however, it has created a need to protect these components against a reverse-connected battery. Many semiconductor devices such as microcomputers, for example, contain a diode connected between the supply voltage and ground. When the battery is properly connected (i.e., with the negative terminal connected to chassis ground) these diodes are reverse biased. When the battery is reversed, however, these diodes become forward-biased. The resulting high currents will almost certainly destroy the device. Thus, all such devices must be protected against a reverse-connected battery.
In many motor vehicles a Schottky diode is connected in series with the load, so that it is forward-biased when the battery is properly connected. If the battery is reversed, the diode becomes reverse-biased and the loads are protected from reverse currents or negative voltages. A disadvantage of this technique is that during normal operation a voltage drop exists across the forward-biased diode and heat energy is generated. For example, a 60 volt Schottky diode might generate a 0.8 volt drop, and with a current flow of 20 amps about 14 watts of heat energy would be generated. This heat must be transferred away from the diode. Heat sinks are becoming more difficult to find in motor vehicles, however, because more parts are being made of plastic. The metal surfaces in the engine compartment are generally too hot to serve as heat sinks.
The most attractive solution to this problem would be a device which approximates as closely as possible an ideal diode, with an equivalent resistance in the forward direction of no more than 50 milliohms. One possibility would be to connect a low on-resistance N-channel power MOSFET in series with the load, with its source connected to the battery and its drain connected to the load. Properly driven, during normal operation the power MOSFET's low resistance channel would shunt any current away from the intrinsic drain-body diode, producing a low on-state voltage drop. The diode formed by the drain-to-body junction is in parallel to the MOSFET's source-to-drain terminals whenever the source-body short is employed in a MOSFET. The source-body short is common in a vertical DMOS. When the battery is reversed, the MOSFET would be shut off, leaving the intrinsic drain-to-body diode reverse-biased. However, to turn the MOSFET on, its gate must be biased at least 8 volts above its source, which is connected to the battery. A charge pump or similar means is necessary to produce a gate-source voltage of a magnitude sufficient to guarantee that the MOSFET will be fully turned on. This charge pump must therefore have one terminal connected directly to the battery.
If a semiconductor device is used to provide the gate-source voltage, this device is susceptible to the same reverse-battery problems as the other devices in the motor vehicle. That is, if the gate driver is connected directly to the battery's plus and minus terminals (i.e., between the vehicle's "hot" and ground terminals), it will be subjected to a reverse potential if the battery is hooked up in reverse. In many self-isolated and junction-isolated IC technologies, a reverse battery connection will forward-bias many junctions in the IC, flood the substrate with minority carriers, or cause latch-up, excessive heating or other undesirable effects.